Performing health physics and radiation safety functions under a special nuclear material license and a research and test reactor license at a major government re- search and development laboratory encompasses many elements not encountered by industrial, general, or broad scope licenses.
Reactor Health Physics
Operations at the NIST Center for Neutron Research
Thomas P. Johnston*
Abstract: Performing health physics and ra- diation safety functions under a special nu- clear material license and a research and test reactor license at a major government research and development laboratory encompasses many elements not encountered by industrial, general, or broad scope licenses. This article reviews elements of the health physics and radiation safety program at the NIST Center for Neutron Research, including the early his- tory and discovery of the neutron, applica- tions of neutron research, reactor overview, safety and security of radiation sources and radioactive material, and general health physics procedures. These comprise precau- tions and control of tritium, training program, neutron beam sample processing, laboratory audits, inventory and leak tests, meter cali- bration, repair and evaluation, radioactive waste management, and emergency response.
In addition, the radiation monitoring systems will be reviewed including confinement build- ing monitoring, ventilation filter radiation monitors, secondary coolant monitors, gas- eous fission product monitors, gas monitors, ventilation tritium monitor, and the plant effluent monitor systems. Health Phys. 108 (Supplement 1):S19–S28; 2015
Key words: operational topics; instrumen- tation; neutrons; reactor, nuclear
INTRODUCTION
The National Institute of Stan- dards and Technology (NIST) NIST Center for Neutron Research (NCNR) is part of NIST at Gaithersburg, Maryland. The efforts at NCNR focus on providing neutron mea- surement capabilities to the U.S. re- search community. The NCNR is a national research center that uses thermal neutrons and cold neu- trons, offering facility instrumenta- tion for qualified applicants. Many of the instruments rely on intense beams of cold neutrons emanating from an advanced liquid hydrogen moderator (Cappelletti et al. 2001).
Neutron beams at the NCNR have the characteristics of being small in size (cross-section typically less than 2 cm 4 cm), high intrabeam dose rates (may be greater than 1 Sv h−1),
and low dose rates outside the beam.
Control measures at NCNR include signage on entrance doors for areas that contain neutron beams. The neutron beam areas are High Radia- tion Areas as defined by U.S. Nu- clear Regulatory Commission (U.S.
NRC) 10 CFR 20 (U.S. NRC 2013).
Every neutron beam area has sim- ilar identifying features includ- ing neutron beam “On or Off”
signs, posted neutron beam warn- ing signs, and alarming infrared proximity detectors (NCRP 1971).
Neutrons are powerful probes of the structure and dynamics of materials ranging from molecules inserted into membranes that mimic cell walls to protons that migrate through fuel cells. The unique properties of neutrons can be exploited by a variety of measurement techniques to pro- vide information not available by other means.
At the NCNR, neutrons reveal properties not available to other probes used in scientific research.
Neutrons can behave like micro- scopic magnets, can diffract like waves, or set particles into motion as the neutrons recoil from the par- ticles. Wavelengths of neutrons range from≈0.01 nm (thermal) to
≈1.5 nm (cold) (1 nm = 10 Å), allowing them to form observable ripple patterns from structures as
*National Institute of Standards & Technology, 100 Bu- reau Drive, MS 6100, Gaithersburg, MD 20899‐6100.
The author declares no conflicts of interest.
Thomas P. Johnston, RRPT, Health Physicist, is a member of the Reactor Health Physics Group at the National Institute of Standards & Technology and the NIST Center for Neutron Research. Tom previously held a faculty position in Physiology at New York Medical College and served as the RSO. Beginning in 1975, Tom was exposed to the field of radiation safety and health physics as a junior in high school while working in Radiology and Nuclear Medicine at the Biloxi Regional Medical Center. Tom served in the U.S. Navy Nuclear Power program, earned a BS in radiation protection from Thomas Edison State College, is certified by the National Registry of Radiation Protection Technologists, and is pursuing certification from the ABHP. Tom is licensed as an inspector for radiation producing machines and high energy radiation producing machines.
Tom has taught graduate courses in radiation safety and radioisotope use in biomedical research. Tom is active in the Health Physics Society and the Baltimore-Washington Chapter. Tom enjoys time with his family, and is an avid fisherman and sailor.
Operational Topic
large as proteins to as small as atoms (Cappelletti et al. 2001). Neu- tron energies are on the scale of millielectronvolts, the same as the motions of atoms in solids or liquids, waves in magnetic mate- rials or vibrations in molecules.
Exchanges of energy between neutrons and matter as small as nanoelectronvolts and as large as tenths of electronvolts can be de- tected. Selectivity in scattering power varies from nucleus to nu- cleus almost randomly. Specific isotopes can stand out from other isotopes, even of the same kind of atom. Specific light atoms, dif- ficult to observe with x rays, are revealed by neutrons. Hydrogen, especially, can be distinguished from chemically equivalent deu- terium. Magnetism makes the neutron sensitive to the magnetic spins of both nuclei and elec- trons, allowing the behavior of ordinary and exotic magnets to be detailed precisely. Neutrality of the uncharged neutrons al- lows the neutrons to penetrate deeply without destroying sam- ples and pass through walls con- trolling a sample’s environment allowing measurements under ex- treme conditions. Properties rang- ing from residual stresses in steel girders to the unfolding motions of proteins are amenable to mea- surement by neutrons. The charac- teristic radiation emanating from specific nuclei will capture inci- dent neutrons and these can be used to identify and quantify min- ute amounts of material in pol- lutants or ancient pottery shards (Cappelletti et al. 2001).
Neutrons are particularly well suited to investigate all forms of magnetic materials such as com- puter storage and memory devices.
The atomic motion of hydrogen can be measured and monitored (in your mind’s eye: picture wa- ter molecules during the setting of cement). Residual stresses trapped inside stamped steel automobile parts can be mapped. Neutron- based research covers a broad
spectrum of disciplines, includ- ing engineering, biology, mate- rials science, polymers, geology, chemistry, and physics.
The NCNR's neutron source provides intense beams of neu- trons required for these mea- surements. In addition to the thermal energy neutron beams from the heavy water moderator, the NCNR has a liquid hydrogen moderator (cold source) that sup- plies intense neutron beams for high resolution research at the cold neutron facility.
There are 28 experiment sta- tions that provide high neutron flux positions for irradiation, and neutron beam facilities used mostly for neutron scattering research.
The NCNR provides neutron research facilities for researchers from industry, university, and government agencies. These fa- cilities are operated with many different modes of access. The fa- cility has major instruments available either via a scientific proposal review program, collab- orative research with a NCNR research scientist, or on a com- mercial basis for confidential R&D. There is no access charge for research for which results are freely available to the gene- ral public. A monetary assistance program is available to encour- age first-time users to do mea- surements at our facility.
The NCNR supports impor- tant NIST research needs, and is also operated as a major natio- nal user facility with merit-based access made available to the en- tire U.S. technological commu- nity. Every year, more than 2,000 research participants from across the country, including industry, academia, and government use the facility for measurements.
Access is gained through a peer- reviewed, web-based proposal system with beam time allocated twice a year by the Beam Time Allocation Committee (Cappelletti et al. 2001).
The mission at NCNR is threefold:
1. Safely operate the NCNR as a cost-effective national resource;
2. Conduct broad research pro- grams using neutron tech- niques, and to develop and apply new neutron measure- ment techniques; and
3. Operate the NCNR as a na- tional resource for researchers from industry, university and government agencies.
Reactor overview
The 20 MW NIST research reac- tor provides a source for neutron scattering techniques. The Neutron- Beam Split Core Reactor (NBSR) is located at the 1,384 hectare site purchased in 1959 for the new lo- cation of the National Bureau of Standards (NBS) in Gaithersburg, MD. In 1988, the name of the NBS was changed to NIST (Rush and Cappelletti 2011).
The NBSR is a heavy water mo- derated and cooled enriched fuel, tank type reactor designed to oper- ate at 20 MW of power. The reactor is a custom designed variation of the Argonne CP‐5 class reactor.
The difference from the CP‐5 in- clude power rating, core configu- ration and cold neutron source.
The major modifications to the CP‐5 basic design are the 18‐cm gap between the upper and lower fuel regions in each fuel element to reduce the fast neutron back- ground in the neutron beams;
a double plenum at the bottom of the reactor vessel to provide optimized cooling to the reactor core; and the method for remote handling of fuel elements during refueling operations. The maxi- mum thermal neutron flux near mid-plane is on the order of 1014 neutrons cm−2s−1at 20 MW. The NBSR is operated, maintained and refueled by NRC licensed senior re- actor operators. The typical cycle for reactor operations is 38 opera- tional days (24 h d−1) followed by 10‐d shutdown for refueling and maintenance activities.
The basic nuclear reaction in heavy water is slow; that is, the prompt neutron lifetime is rela- tively long and reactivity coeffi- cients of temperature and void are negative. The reactor operates in a low temperature unpressur- ized condition and has no large stored energy content. The maxi- mum hypothetical accident as- sumes complete melting of one fuel assembly. For this scenario, the dose to a person on the site boundary 24 h a day for 30 d would be 0.07 mSv and the iodine dose to the thyroid would be neg- ligible (1μSv). Inherent (or passive) safety features include:
• The reactor core is designed so that the temperature coeffi- cient of reactivity is negative;
• The reactor core is designed so that the void coefficient of reactivity is negative; and
• There is a passive gravity drain of 3,000 L of D
2O from the in- ner reserve tank within the re- actor vessel into the core.
Neutrons generated via nu- clear fission have high kinetic en- ergies, and are slowed to thermal energies by heavy water surround- ing the fuel. Some neutrons are slowed even further by a volume of liquid hydrogen (cold source) located near the fuel. The neu- trons produced are called thermal neutrons and cold neutrons re- spectively. In addition to scatter- ing, which is the main technique used at the NCNR, neutrons can be used to create new stable or radioactive nuclei by absorption.
This property is the basis of Neu- tron Activation Analysis, a tech- nique used in analytical chemistry and health physics for identifica- tion and quantitative analysis of isotopic species. Other measure- ments at the NCNR involve the fundamental properties of the neu- tron, such as neutron lifetime (free neutrons are unstable and decay with a mean lifetime of about 15 min into a proton, an electron,
and an antineutrino). Typical re- search conducted at the NCNR in- volves the analysis of materials to be utilized in, for example:
1. magnetic monopoles;
2. magnetic data storage;
3. high temperature superconduc- tivity;
4. energy storage and collection devices (batteries, fuel cells and solar cells);
5. piezoelectric materials;
6. biological applications; and 7. polycrystalline modeling.
This article will focus on the aspects of applied health physics utilized for radiation safety and security of radioactive material at the NCNR (Shleien et al. 1998).
Radiation and radioactive material:
safety and security
Access to the NIST campus is controlled. Admission to the NCNR facility is controlled and right of entry to the research areas has additional access and security measures. Table 1 delineates how the entrances to research areas are posted. Each instrument experimen- tal area has specific access controls and security and safety measures in place. Instrument areas are posted as Radiation Area or High Radiation Area as needed (U.S. NRC 2013).
During periods when the reac- tor is critical, daily radiation surveys are performed with a pressurized ion chamber to assess adequacy of posting and control measures.
Fig. 1 shows the instrument loca- tions where the daily radiation readings are taken and represents
an overhead view of the facility and the locations of data collec- tion points. The distance from the reactor center to the beam stop on NG‐6 End Station is 150 m.
Table 2 lists typical location sampled, maximum, mean, and minimum radiation level in units ofμSv h−1 for the daily radiation readings collected. Table 2 repre- sents a review of data collected during a prior year of reactor op- eration at the NCNR. A pressur- ized ion chamber was utilized to collect this data. The C100 loca- tions represent data collected in the reactor building at the ther- mal neutron beam instruments.
The Guide Hall locations repre- sent data collected at the cold neutron beam instruments and experiment locations. Routine wipe tests are conducted to ensure in- strument experiment areas remain free of loose radioactive material.
Throughout the facility wipe tests are performed and analyzed for re- movable radioactive material with gas flow proportional counting sys- tem. The samples may also un- dergo liquid scintillation analysis and gamma spectrum analysis.
All personnel that enter Con- trolled Areas (Reactor confine- ment and Guide Hall areas) upon exiting must pass through radia- tion portal monitors each with six large area (27.9 cm 48.3 cm) gamma-sensitive plastic scintilla- tor detectors. Half-body monitors (with gas flow detectors) and Hand and Foot monitors (with sealed proportional detectors) are stra- tegically located at exit points throughout the building adjacent to research laboratories and op- erational support facilities. Area radiation detectors are in the Reac- tor building and the Guide Hall.
Alarming continuous air monitors that sample particulates and noble gasses are located throughout the facility. Routine and non-routine grab air samples are collected with evacuated ionization chambers. Per- sonnel are monitored with self- reading pocket ion chambers (PICs)
Table 1. Research area entrance postings.
Research Area Entrance Postings Restricted Area: Dosimetry Required Area,
Visitors must be escorted
Caution: Radioactive Materials work area CAUTION: High Radiation Areas in the
beams in this room are designated by NEUTRON BEAM—NO ENTRY, contact Health Physics for additional information
No eating, drinking, smoking in this area
and four element thermolumines- cent dosimeters (TLDs). The TLDs (a high-sensitivity LiF:Mg,Cu,P ther- moluminescent material, TLD‐600/
700; Note: TLD‐600 contains 4.40%
7Li and 95.60% 6Li while TLD‐700 contains 99.93%7Li and 0.07%6Li) monitor photon Deep Dose, Shal- low Dose, Eye Dose, and albedo Neutron Dose. Albedo TLD’s mea- sure scattered thermal neutrons emitted from the body. These do- simeters also measure incident ther- mal neutrons since no cadmium was used in the filtration of the TLD holder to absorb thermal neu- trons (Cassata et al. 2002). Visitors are monitored with pocket ion chambers that contain TLDs (TLD chips are attached to the outside of the PIC with heat-shrink tubing).
Personnel that routinely enter cer- tain areas must submit monthly urine samples for bioassay analy- sis. Health Physics (HP) and Reac- tor Operations staff is monitored with a baseline urinalysis and whole body count upon initial assignment at the NCNR. Environmental TLDs are located inside and outside the
facility and along the NIST fence.
Environmental radiation data are collected monthly from battery powered Geiger-Mueller (G-M) de- tectors located inside and outside the facility and along the NIST fence.
Environmental vegetation and soil samples are collected monthly on the NIST campus and undergo gamma spectrum analysis. Environmental water samples are collected monthly from locations on and off the
FIG. 1. Location of data collection points.
Table 2. Facility radiation readings inμSv h−1.
C100 locations Maxa Meana Mina
BT-5 81.0 31.1 2.3
BT-4 350.0 74.7 1.0
BT-2 450.0 40.7 0.3
BT-1 98.0 28.5 1.5
BT-9 400.0 68.4 0.8
BT-8 192.0 27.4 3.0
BT-7 1,080.0 236.6 1.2
MACS 2,000.0 120.1 1.1
Guide Hall locations Maxa Meana Mina
NG-7 Reflectometer 98.0 36.0 21.0
NG-7 Interferometer 45.0 9.8 1.0
NG-7 Prompt Gamma 181.0 31.9 1.5
NG-7 30 M SANS 124.0 32.7 0.1
NG-6 End Station 46.0 8.5 0.1
NG-6 U (Ultra Cold) 12.2 1.9 0.1
NG-6 Monochromator Beam 34.0 16.2 1.5
NG-6A (Lyman/Alpha) 68.0 20.0 12.0
NG-5 Spin Echo 280.0 75.7 1.5
NG-5 SPINs 65.0 6.5 2.0
NG-4 Disk Chopper TOF 120.0 14.3 0.2
NG-3 30 M SANS 87.0 12.8 0.5
NG-2 Backscatter 90.0 19.4 1.3
NG-1 NDP 140.0 45.6 1.5
NG-1 Reflectometer 21.0 2.4 1.0
NG-1 AND/R 16.1 6.6 0.6
aReadings inμSv h−1taken with pressurized ion chamber.
NISTcampus and undergo liquid scintillation and gamma spectrum analysis (Cassata et al. 2002; NCRP 1971, 1976b, 1986).
General health physics procedures The general health physics procedures provide for protection of NIST personnel and the public, to comply with applicable regu- lations and the NBSR reactor and radioactive material licenses and to comply with management pol- icy and commitment to keep ra- diation exposures and releases as low as is reasonably achievable (ALARA). All individuals have the responsibility to protect them- selves, to maintain their radiation exposures ALARA, and to actively ensure that NBSR ALARA goals are met. During normal business hours, the NBSR HP staff is avail- able to assist in matters pertaining to the radiation protection pro- gram and for matters involving radiation or radioactive material.
The Duty HP is on call 7 days a week and 24 hours per day. Dur- ing off hours reactor operators are available to handle immediate needs of visitors and staff, with the Duty HP on call. The National Institute of Standards and Tech- nology, the management of the NBSR, and the HP staff are com- mitted to the policy to keep ex- posures within the limits defined by NRC 10 CFR 20, and to keep exposures as low as reasonably achievable consistent with sound operating practices (U.S. NRC 2013). All operations are planned and conducted in a manner con- sistent with this policy. Approval by the NCNR Director and HP is required prior to bringing radioac- tive materials into the NCNR. HP approval is required for transfers of radioactive material from the NBSR to other NIST areas and off site facilities in accordance with U.S. Department of Transporta- tion (U.S. DOT) and International Air Transport Association (IATA) regulations (NCRP 1978; U.S. DOT 2012; IATA 2012).
The ALARA program required by 10CFR20.1101(b) is integrated into these procedures (U.S. NRC 2013). Elements of the program in- clude: strong supervisory oversight of operations involving potential significant exposures; explicit, re- viewed procedures via Radiation Work Permits (RWP) for major ac- tivities; review of work experiences and exposure histories; individual involvement and responsibility for meeting ALARA goals; and, review of environmental sampling and dosimetry system results. HP con- ducts an annual review of HP pro- cedures and the implementation of 10 CFR Parts 19 and 20 (U.S.
NRC 2012, 2013). The review em- phasizes a continuing application of ALARA principles, current pro- tection practices, and application of experience gained at the NBSR and other facilities (Emery et al.
1995). The greatest contributor to reduction in worker exposures dur- ing maintenance activities is di- rectly related to HP oversight.
HP direct oversight results in re- duced personnel exposures espe- cially during activities dictated by RWP.
Precautions and control of tritium Precautions to prevent or minimize exposure to tritium in- clude monitoring the air with tri- tium monitors, with cold trap samples of water vapor evaluated in a liquid scintillation counter, and with air samples collected in an evacuated ionization chamber and evaluated with an electrome- ter; monitoring surfaces with smears or wipes counted in a liquid scin- tillatio counter; restricting exposure time for work in elevated tritium concentrations; using a local ex- haust system for ventilation around maintenance operations on the D2O and helium systems; and re- quiring waterproof clothing be used to avoid direct skin contact with water containing high levels of tri- tium. Tritium uptake is determined by urinalysis, using a liquid scintil- lation counter. Routine and timely
samples are required from persons working with tritium or tritium- contaminated materials. Personnel designated by HP regularly submit a urine sample each month. A spe- cial sample is submitted no sooner than one hour after a known or suspected uptake of tritium or when requested by HP. A special sample is submitted in any week that work is performed involving primary sys- tem water, e.g., leaking primary sys- tem seals, heat exchanger work, etc.
(NCRP 1976a; Liu et al. 1999; Kase et al. 2003; Turner 2007). During refueling operations, reactor op- erators must submit pre-job and post-job samples.
Training
Before arriving at the NCNR, visitors must register and com- plete a sign-in process. Radiation safety training must be requested.
The training may be completed in advance at the user’s facility, office, or home computer. The vis- itor must obtain a letter of identi- fication from their organization (referred to as the “trustworthy”
letter). If the visitor is a returning user already on the NCNR access list, personal dosimetry can be re- quested in advance. Upon arrival at the NCNR, a visitor must un- dergo HP Radiation Safety training, complete a “Facility User Safety Awareness Checklist,” complete a signed Facility User Agreement, and be assigned and issued personal dosimetry (see Appendix for experi- menter and beam user controls).
The HP staff participates in the training of new Reactor Oper- ators and the biannual training and requalification of Reactor Op- erators. Additionally, Emergency Service staff members consist of Emergency Medical Technicians, fire fighters, and federal police officers and receive biannual train- ing from HP staff. HP staff prepares and delivers additional training at the request of scientific staff. Ex- amples of training topics include glove box operations for neutron beam samples, summer student
introduction to radiation safety, and science teacher introduction to radiation safety.
Neutron beam sample processing Samples or materials that have been subject to a neutron beam are delivered to the HP lab for process- ing, clearance or disposition. The beam user must complete the Ma- terials Transfer Request form for each sample. HP staff performs sample screening. Sample screen- ing consists of appropriate sur- veys with pancake G-M, wipe test and analysis by gas flow propor- tional counter, neutron activation analysis using computer model- ing, and gamma spectrum analysis with high purity germanium de- tector. Sample disposition catego- ries (U.S. DOT 2012; IATA 2012):
1. Release for unrestricted use (no further action needed);
2. Exempt quantity;
3. Radioactive material license transfer (verify receiver ’s li- cense);
4. Not radioactive per DOT or IATA (no transportation re- strictions);
5. Radioactive material per DOT or IATA; and
6. Radioactive waste.
Laboratory audits, inventory and leak tests
The HP staff performs quar- terly audits in labs characterized, commissioned, and posted to pos- sess and utilize radioactive mate- rial. Audits consist of radiation and contamination surveys, hood flow checks, and spot checks of radiation source inventory. The focal points of the Audit include:
1. General neatness and cleanli- ness;
2. Posting and labeling (room, area, cabinets, pygs);
3. Radioactive material security;
4. Food, drink, smoking, control;
5. Radioactive waste manage- ment;
6. Dosimetry usage and storage;
7. Survey instrument availability
and appropriateness;
8. Contamination control (area, personal);
9. Access control;
10. Hood status;
11. Spill containment (benchtop, transport or movement); and 12. Radioactive material usage
locations.
Leak test are conducted in ac- cordance with license requirements for radioactive material. Sample analysis is performed using gas flow proportional counter or liquid scin- tillation counter. Results of de- tected activity of less than 185 Bq are considered acceptable. Prior to receipt of non-exempt radiation sources, a Hazard Review must be completed. These sources are as- signed a Radiation Safety number and entered into the database.
Meter calibration, repair and evaluation
All portable instruments used for the detection of radiation and contamination in use at the NCNR are calibrated at NIST. Calibration is performed with pulser or appro- priate gamma, neutron, or alpha sources. HP staff will troubleshoot and repair meters that malfunc- tion, become damaged or fail cali- bration. Pulsers and flow meters are sent to the vendor for calibra- tion. A small portion of meters must be sent to the vendor for re- pair. Upon receipt of new meters, a calibration is performed prior to placing into service. HP staff has an added position responsibility to keep abreast of developments in the portable meter industry.
This duty entails review of trade literature and obtaining meters for testing and evaluation at the NCNR.
Radioactive waste management The radioactive waste program encompasses a broad range of pro- cesses and materials. Low volume and low activity samples originate from neutron beam-related expe- riments and research. Reactor op- erations and reactor maintenance
events including retooling contribute significant high activity volumes of waste. Liquid waste is collected in onsite radioactive waste tanks and disposed to the sanitary sewer after appropriate sampling and re- view to ensure the radionuclide and activity meet the waste efflu- ent concentration limits in com- pliance with Federal, State, and Local regulations. A commercial waste contractor handles disposal of solid waste and liquid waste not suitable for sanitary sewer dis- posal (NCRP 1996). Waste for ship- ment consists of routine laboratory waste, waste samples from neutron beam experiments, material and equipment from retooling neutron beam lines and neutron beam in- struments. Other high activity waste consists of:
1. Reactor primary resins;
2. Reactor primary filters;
3. Filters and resins from other systems;
4. Shielding plugs and related neutron beam shields;
5. Experiments, or experimental components removed from high neutron flux locations;
6. Reactor components; and 7. Miscellaneous contaminated
materials, such as laboratory waste.
Site emergency response
The NIST Fire Protection Group (FPG) provides around the clock support and protection for the NIST Gaithersburg facility. The FPG pro- vides fire prevention inspections, fire suppression, emergency me- dical service, hazardous materials (HAZMAT), and miscellaneous ser- vices for the Gaithersburg campus.
All firefighters are certified emer- gency medical technicians and hazardous materials technicians.
National Fire Protection Associa- tion (NFPA) codes are accepted as the standard level of protection for the NIST Gaithersburg facility (NFPA 1993, 2014a and b).
The NIST Police Services Group (PSG) provides Federal Police Offi- cers and security staff for police
services at the NIST Gaithersburg campus. The PSG is responsible for the protection of the site.
Radiation monitoring systems Confinement building monitoring. The reactor confinement building monitoring system consists of ten beta-gamma area monitors. The detectors are halogen quenched G-M tubes and each detector as- sembly is equipped with an alarm light and an alarm buzzer. The de- tectors have local dose rate indica- tors with ranges from 0.01μSv h−1 to 1 Sv h−1 (Knoll 1979; Shleien et al. 1998).
Ventilation filter radiation monitors. There are three filter monitors associated with the ven- tilation system: irradiated air ex- haust from the reactor; normal air exhaust (non-irradiated room air taken from the face of the reac- tor); and basement air exhaust.
The activity in these three ducts is indicated on log ratemeters with independently set alarm points. The ratemeters have a range of 20 cpm to 200,000 cpm (cpm, counts per minute) and time constants that vary from 60 s at 20 cpm to 0.05 s at 200,000 cpm. Visual and audible alarms in the Control Room are energized by activity conditions that exceed preset levels on the meter relays. Additional G-M de- tectors are located adjacent to the high efficiency filters in duct- work that serves the laboratory areas. Radiation levels at the fil- ters are indicated on a computer screen in the HP lab. These levels are also recorded in the HP lab.
Count rates that exceed levels preset on the meter relays actuate visible and audible alarms on a computer screen in the HP lab (Knoll 1979; Shleien et al. 1998).
Secondary coolant monitors.
The secondary coolant down- stream from the main D2O heat exchangers is monitored for ra- dioactivity to indicate a primary to secondary leak. The system
has two shielded detectors, two pumps, and associated valves and instrumentation. Normal line-up will have two pumps and both de- tectors in service. Each detector consists of a G-M tube inside a shielded liquid sampler located in an equipment room. The output of these detectors is indicated lo- cally on log ratemeters. Both detec- tor outputs are also indicated and recorded in the Reactor Control Room. The ratemeter has a range of 101cpm to 106cpm, and activ- ity that exceeds preset levels on the meter relay actuates an an- nunciator in the Reactor Control Room. Each pump has the capac- ity to draw secondary water through either detector from the heat ex- changer return line. Water then passes through flow meters, a pH instrument and a test heat ex- changer before joining the return flow to the cooling towers (NCRP 1978, 1986; Knoll 1979).
Gaseous fission product monitors.
The helium sweep gas is continu- ously monitored for radioactivity to provide an indication of a fuel element cladding failure. A sample of helium is pumped through a shielded gas sampler that contains a G-M tube. A log ratemeter, lo- cated with the detector in the monitor room indicates the radio- activity of the helium gas. This level is also indicated and recorded in the Reactor Control Room. The ratemeter has a range of 101 to 106 cpm. Activity that exceeds a preset level on the ratemeter acti- vates a Reactor Control Room an- nunciator. During normal reactor operations at power, on-scale me- ter readings indicate the presence of activated argon, the source of which is in-leakage of air during re- actor shutdown operations, which include maintenance and refueling operations (Knoll 1979).
Gas monitors. The system con- sists of a gas pump, two shielded gas samplers and detectors, and two log ratemeters in a single housing on
the reactor basement mezzanine.
Each detector is a thin-walled sen- sitive G-M tube with the irradi- ated air detector being smaller than the normal air detector. Gas samples are continuously pumped from the irradiated air and nor- mal air ventilation system exhaust ducts. When the activity in either duct exceeds a preset value, a major scram is initiated and au- dible and visual alarms are ener- gized in the Reactor Control Room (NCRP 1986).
Ventilation tritium monitor.
Air within the Confinement Build- ing is continuously sampled for tritium. A flow through ion cham- ber located on the basement level is supplied with air drawn by an associated blower from ten points on the ventilation system. Valved sample points are available for each of the individual sample lo- cations. Tritium concentration is monitored and recorded in the Reactor Control Room, with an annunciator set to alarm at pre- selected levels on the tritium re- corder. A loss of flow through the monitor will also alarm the annunciator (NCRP 1976a).
Plant effluent monitors
Stack gas monitor, RD4‐1. The stack gas monitor consists of one thin-walled sensitive G-M tube lo- cated inside the stack about two- thirds of the way up the stack.
The radiation level is indicated on a log ratemeter in the monitor room and is indicated and recorded in the Reactor Control Room. The rate- meter has a range of 101to 106cpm.
Activity that exceeds a preset level will initiate a major scram and actu- ate the Reactor Control Room an- nunciator (NCRP 1996).
Stack gas monitor, RD4‐2
This monitor consists of one thin-walled sensitive G-M tube located in the stack immediately below the RD4‐1 detector tube.
The RD4‐2 detector tube is smaller than the RD4‐1 detector tube. The
result is that the RD4‐1 detector is more sensitive at the low end of the scale, but the RD4‐2 detector tube is less likely to saturate at the high end of the scale. The ra- diation level is indicated in the Reactor Control Room, with me- ter face and scale different from the RD4‐1 meter. The RD4‐2 me- ter indicates in counts per minute at the low end of the scale and general levels for the rest of the scale. The purpose of this instru- ment is to supply the reactor ope- rations staff with information to assist in the determination of emer- gency conditions, as defined in the NBSR emergency procedures.
CONCLUSION
Neutron beams at the NCNR research reactor, a major govern- ment research and development laboratory, present a sufficient va- riety of applied and operational health physics applications that requires continuous oversight to maintain safety, control, and se- curity of radioactive material and radiation sources. An introduc- tion as to why neutrons are used in research and development was presented. Many aspects, includ- ing general health physics proce- dures; training; precaution and control of tritium; neutron beam sample processing; lab audits, in- ventory and leak tests; meter cal- ibration, repair, and evaluation;
waste management; emergency re- sponse; and the radiation monitor- ing systems must be incorporated into the health physics program.
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APPENDIX
General HP experimenter controls at the NCNR
1. All individuals have the respon- sibility to protect themselves, to maintain their radiation exposures ALARA, and to actively ensure that NBSR ALARA goals are met. Always consult HP (or the Reactor Supervisor in their absence) whenever there are questions concerning radiation exposure, contamination or other radia- tion protection problems;
2. Eating, drinking, and the stor-
age of food are not permitted
in the confinement building
(except the reactor control room
and operator office areas), in
the Guide Hall, or in any lab-
oratory used for unsealed ra-
dionuclide work. Smoking is
prohibited;
3. Notify HP or the Reactor Su- pervisor immediately of any accident or injury involving radiation or radioactive ma- terial;
4. All experiments shall be shielded so that in any acces- sible area, the radiation level is less than 1 mSv h
−1at 30 cm from any surface. HP shall be contacted prior to conducting any experiment that might create a High Ra- diation Area. High Radia- tion Area posting and control requirements shall be imple- mented and a Radiation Work Permit may be required;
5. A radiation sign, rope or bar- rier shall not be removed without consulting HP. If it is necessary to move a radia- tion rope or barrier temporar- ily for equipment movement, then that area shall be attended.
The radiation rope or barrier shall be replaced in its origi- nal position when finished;
6. Portable radiation survey in- struments located on each floor should be used for short duration surveys and promptly returned to their station. Addi- tional survey instruments may be obtained from HP for lon- ger use;
7. A personal contamination check shall be performed each time upon leaving B-wing (warm lab or hot cell area), C-wing, or G-wing;
8. Radiation dosimeters shall be worn at all times in areas posted as requiring personal dosimetry;
9. Direct-reading pocket dosime- ters shall be worn when work- ing with radioactive samples or inside a posted Personnel Monitor Required Area or High Radiation Areas. Dosim- eters shall be checked periodi- cally by the wearer to ensure that exposure limits are not exceeded;
10. Return radiation badges and dosimeters to the racks upon
leaving the building. Pocket dosimeter readings are peri- odically checked by HP to determine the accumulated dose;
11. Personnel working with high radiation level components or samples shall contact HP for supplementary dosime- try, e.g., finger dosimeters;
12. All radioactive material be- ing removed from Building 235 shall be properly pack- aged and surveyed by HP, Reactor Operations, or other authorized personnel and all required forms completed.
(See license SNM –362 and implementing procedures for transfers within NIST;
†) 13. All radioactive items or con-
tainers of licensed radioac- tive material shall bear a durable, clearly visible label identifying the radioactive contents. The label shall bear the radiation caution symbol and the words "Caution, Ra- dioactive Material" or "Dan- ger, Radioactive Material". It should also provide sufficient information to permit indi- viduals handling or using the containers, or working in the vicinity thereof, to take pre- cautions to avoid or minimize exposure;
14. In the event of a local radiation alarm, leave immediate area, and notify HP or Control;
15. Response to fire and evacua- tion alarms shall be the same as the rest of Building 235.
Leave by the nearest exit
and assemble in front of the building;
16. If it is necessary to open out- side access doors to the Guide Hall (G-wing) for any duration, the open door shall be monitored by a person on the access list. That person is responsible for any person entering the building. Out- side access doors to the Con- finement Building (C-wing) shall not be opened without prior approval from the Re- actor Supervisor. The North and South access doors to the Confinement Building shall not be blocked;
17. All new experiments will ini- tially be surveyed by HP if re- quired after review by the SEC and approval by the Di- rector, NCNR. Changes in procedures or shielding may be recommended at this time.
After changes are made, the experiment will be resur- veyed until satisfactory; and 18. HP shall be notified of any sig- nificant change made to the experiments so that they may be resurveyed if necessary.
Beam user controls at the NCNR
1. All required safety systems for a specific experiment shall be present and opera- tional unless a specific waiver is obtained from HP.
HP shall be immediately no- tified of any failure or mal- function of such a system, e.g., interlocks, warning de- vices, etc.;
2. All external beams from the Reactor shall be properly barricaded to prohibit person- nel from entering the beam accidentally. Beams with an accessible length greater than 30 cm are High Radiation Areas. Designed access con- trols (e.g., beam status in- dicators, barriers, local signs indicating beam status, intru- sion detectors, or experiment
†NIST establishes and maintains an ion- izing radiation safety program at the NIST- Gaithersburg site in accordance with Nuclear Regulatory Commission (NRC) License Num- bers SNM‐362, 19‐23545‐01E, and TR‐5 (Test Reactor) and applicable Federal, State, and lo- cal regulations. The program has two functional areas: Radiation Facilities (e.g., laboratories containing radioactive materials, gamma irradi- ators, accelerators, electron microscopes, and x-ray devices) and the Reactor Facility at the NCNR. NIST Radioactive Material Licenses, 19‐23545‐01E, SNM‐362. NIST Reactor Li- cense, TR‐5.
features minimizing accessi- bility) must be functional.
Exposure to these radiation beams shall be avoided at all times. Any suspected expo- sure shall be reported to HP or the Control Room imme- diately;
3. HP or Reactor Operations shall be contacted prior to any work to be done inside the biological shield with the beam shutter open. A Radiation Work Per- mit may also be required;
4. Any components removed from an experiment shall be checked for activation or contamination. All such ra- dioactive items shall be ap- propriately labeled and shall not be transferred to an un- restricted area without HP approval. Any components removed from the biological shield or a direct neutron beam shall be surveyed by HP, Reactor Operations, or other authorized personnel;
5. For experiments requiring cooling where the coolant could become radioactive, the integrity of the cooling system shall be maintained to prevent spillage and mini- mize radioactive gases vented to the irradiated air system;
6. Systems or controls labeled with the "NBSR - DANGER - DO NOT OPERATE" red tag shall not be operated or disturbed without specific authorization from the Reac- tor Supervisor; and
7. All users of reactor beams shall receive training on the safety and operation of the specific beam tube and ex- perimental facility used.
In-core irradiation user controls at the NCNR
